There are many situations where there is a requirement to deliver therapeutic agents to specific targets within the brain parenchyma via implanted catheters. Furthermore, many of these therapeutic agents will cause unwanted side effects if delivered to healthy parts of the brain. Examples of treating abnormalities of brain function include the acute infusion of Gamma-aminobutyric-acid agonists into an epileptic focus or pathway to block transmission, and the chronic delivery of opiates or other analgesics to the periaqueductal grey matter or to thalamic targets for the treatment of intractable pain. Also, cytotoxic agents can be delivered directly into a brain tumour, intraparenchymal infusion can also be used to deliver therapeutic agents to brain targets that can not be delivered systemically because they will not cross the blood-brain barrier. For example, the treatment of patients with Parkinson's disease, Alzheimer's disease, head injury, stroke and multiple sclerosis maybe carried out by the infusion of neurotrophic factors to protect and repair failing or damaged nerve cells. Neurotrophins may also be infused to support neural grafts transplanted into damaged or malfunctioning areas of the brain in order to restore function. It is also known to insert instruments other than catheters, such as electrodes, directly in the brain parenchyma. For example, stimulating and lesioning electrodes are used in a variety of surgical procedures, including deep brain stimulation (DBS) electrodes. A surgeon wishing to stimulate or lesion a particular area of nervous tissue can target the end of an electrode to the target site so that a desired electrical current can be delivered.
Once a catheter or electrode has been inserted into the target area, it may need to remain in place for some time. It would be useful to be able to substantially seal the channel through which the instrument has been passed, so as to reduce the likelihood of the ingress of micro-organisms etc. it would also be advantageous to be able to remove the instrument without opening the seal. In addition, it is often necessary to administer a course of treatment to the same area repeatedly and so a surgeon may need to access the patient's brain through the same channel on a number of occasions. It would be helpful to be able to do this, without repeated opening of the channel surgically.